1. Field of the Invention
The invention relates to a carrier for magnetic data formed by perpendicular recording.
2. Description of the Prior Art
To record magnetic data on a track of a data carrier, there are three uniaxial methods of recording. These methods are identified by respective ones of the three dimensions of the track. In longitudinal recording, the magnetic fields representing the data (termed magnetic data fields) extend in the lengthwise direction of the track. In transverse recording the fields are perpendicular to the lengthwise direction of the track in the plane of the carrier. In perpendicular recording, the magnetic fields are perpendicular to the track and to the plane of the carrier. There is also a fourth, circular method of recording magnetic data. The circular recording method is characterized by closed circular fields in the longitudinal plane normal to the plane of the carrier.
The most widely used form of data carrier is magnetic tape, but increasing use is being made of magnetic discs, especially in data processing.
Magnetic recording, in the perpendicular mode on strips of paper, began in about the year 1920. However, it was soon superseded by longitudinal recording in view of the ease with which the latter could be performed, its reliability, and the simplicity of the equipment involved in the reading and writing of data. Longitudinal recording has found wide acceptance. Transverse recording is more difficult to put into practice and therefore has only a few very special applications. Circular recording is not used industrially.
To give a better idea of the actual advantages and disadvantages of the three main methods of recording magnetic data, the following description will relate to the recording of digital data as an example. Items of data of this kind are each contained in successive regions of a track termed "cells". To conform to the laws of magnetism, the neighboring fields of successive cells are directed in opposite directions, no matter what the method of recording. Zones termed "transitions" thus exist between the cells. These transitions are of course the site of considerable magnetic variations which produce strong demagnetizing fields. The different values of the items of digital data are usually represented either by cells of different lengths or by the magnetic complexity of the cells, as is the case with the Aiken code for example, also termed the "dual frequency code", in which an item of 0 data is represented by a cell having only a single magnetism and an item of 1 data by a cell consisting of two half-cells having opposite fields.
In the following description, d will indicate the length of a cell, e will represent the thickness and w the width of a cell and t will denote the length of a transition.
In longitudinal recording, the length t of each transition is related, by complex functions, to the magnetic properties and thickness e of the layer which forms the track, and to the spatial distribution, in the layer, of the field produced by the head. It follows from this that the transitions t may be of greater or lesser extent relative to the length d of the cells. Merely from the point of view of the space occupied, the said extent prevents recording with a high data density. However, when the length t of the transition is equal to or exceeds the length d of a cell, the magnetic layer is substantially demagnetized, and as a result the leakage flux becomes very low and inadequate to enable data to be detected and decoded. It should be added that the track, when seen through the electron microscope, has transitions t which are not straight, but of a sawtooth configuration, which to all intents and purposes increases their size still further in relation to the effective length d of the cells. The longitudinal method of recording is not suitable for obtaining the higher data densities required in particular for data processing, and effort has therefore been concentrated on the other two methods.
The advantage of transverse recording derives from the fact that the length t of the transitions is extremely small since it typically forms walls of the Neel type familiar in magnetism, given the relatively low order of magnitude of the thickness e of the track. Unfortunately, attendant on this fact, which is favorable to high recording densities, is the need to use on the one hand tracks of a soft, anisotropic magnetic material to obtain an anti-parallel orientation of the cells and to assist in writing, and on the other hand heads of complicated structure which generate a weak writing field. The writing and preservation of data are thus very much affected by external interference fields and so represent operations which are difficult to perform (see for example U.S. Pat. No. 3,611,417).
Perpendicular recording likewise has the advantage of creating narrow transitions, of which the characteristics should theoretically approximate to those of Bloch walls, but have as yet been little explored experimentally. It is all the more effective the higher the recording density. In effect, the shorter are the cells, the stronger the coupling between cells. However, the uncompensated demagnetizing fields H.sub.d which appear at the surfaces of the tracks conform to the formula H.sub.d =Md/e, where M is the magnetization vector and d and e are the length and thickness of a cell. This formula demonstrates that perpendicular recording is all the more favorable when d is low and e is high. However, the thickness of the track cannot be increased as desired since this would produce an undesirable increase in the divergence of the write fields and would thus reduce the definition of the cells which, given their small length d, becomes an important factor which must be respected. Otherwise, the way in which this method can be implemented is already familiar for recording on tape but it is not yet known for recording on magnetic discs.
The distinction between tapes and discs which makes it difficult to employ the perpendicular method is due to the difference which generally exists between the nature of the substrates of tapes and discs. "Substrate" refers to the member which carries the magnetic tracks.
In the case of tape, the substrate is generally an electrically insulating strip which is thin (typically of the order of 5 .mu.m), especially in its particular use for high densities, as in data processing for example. In this way, a magnetic head whose two pole-pieces are arranged on either side of the strip, thus enclosing the strip in its air-gap, is perfectly suitable and adequate to create perpendicular fields properly and easily. Owing to the small thickness of the tape, the air-gap remains of the small dimensions which is conducive to the efficiency of the head and to the definition of the written data. Also, the electrically insulating material which forms the substrate cannot give rise to eddy currents capable of upsetting the desired fields which are handled by the head.
On the other hand, the substrate of a conventional present-day magnetic disc is thick (of the order of 1 to 2 mm), and is made of a non-magnetic conductive material (generally aluminum). Such discs often have data recorded on both faces, so that the width of the air-gap and, in particular, the eddy currents and the fact of recording on both faces give rise to new problems to be solved not present with respect to magnetic tape.
Attempts so far made to solve these problems have not produced a valid solution.
As an example, an attempted solution described by the Japanese S. Iwasaki and Y. Nakamura, in the journal "IEEE Transactions of Magnetics" vol. MAG-13, No. 5, September 1977, pages 1272 to 1277, although original and interesting, is nevertheless restricted to application to tapes, by reason of the fact that the write field still has to pass through the substrate. However, because on the one hand of the anisotropy of the magnetic layer, which is formed by high-frequency sputtering of a chrome-cobalt compound and which is orientated in such a way that the axis of easy magnetization is perpendicular to the plane of the substrate (a thin film of polyimide), and because on the other hand of the special single-pole head (through whose air-gap the carrier passes), the field lines are concentrated in the magnetic layer and thus provide high recording density and good definition. It should however be noted that there are a number of known methods of depositing layers having perpendicular anisotropy and that a large number of compounds are known which can be used to form them, as is described, for example, in French Patent No. 2,179,731.